This invention relates to an audio muting apparatus for muting an audio signal under the control of means for interrupting passage of the audio signal using a D/A converter.
In a television receiver, when the channel to be received is changed, an audio signal is muted. A conventional technique for muting is disclosed in the Unexamined Japanese Utility Model Application Publication No. Sho 63-65332. Specifically, in this technique, a pulse creating means is provided which creates a train of pulses which have a shortest or longest width at a starting time or ending time of muting and an opposite width at an intermediate time. A low pass filter is also provided for converting the pulse train into a muting signal having a gentle slope. In response to the muting signal, the muting means mutes the audio signal.
However, the above technique gives rise to the following problem. Specifically, in order to convert the pulse train created by the pulse creating means into the muting signal with a gentle slope, the low pass filter must be provided. On the other hand, the low pass filter must be constructed using discrete components such as a resistor and capacitor. This presented the problem of an increase in the number of discrete components. Further, in order to change the width of the pulse width of the pulse train, complicate control is required. This presented the problem of complication of the program for control.
This invention has been accomplished in order to solve the above problem.
An object of this invention is to provide an audio muting apparatus which can omit a low pass filter by creating a signal having a gentle slope necessary for the muting using a D/A converter.
Another object of this invention is to provide an audio muting apparatus which can attenuate the level of pop sound in releasing the muting to a practical level by setting the converting accuracy in the D/A converter to the value exceeding 8 bits.
Still another object of this invention is to provide an audio muting apparatus which can attenuate the level of pop sound more greatly by decreasing the varying speed of the output voltage from the D/A converter when a muting transistor is shifted from an xe2x80x9cONxe2x80x9d state from an xe2x80x9cOFFxe2x80x9d state.
A further object of this invention is to provide an audio muting apparatus for preventing complication of the control for muting without generating pop sound in such a manner that the time interval when the state of a D/A converter is changed between a first output state and a second output state is fixed whereas the difference between the output voltage in the first output state and that in the second output state is varied and the envelope signal of the output from the D/A converter is taken out as a muting signal.
A still further object of this invention is to provide an audio muting apparatus which can decrease the capacitance of a capacitor in a low pass filter by shortening the time interval when the state of a D/A converter is changed between a first output state and a second output state.
A further object of this invention is to provide an audio muting apparatus which can mute sound when the channel to be received is changed.
In order to solve the above problem, the audio muting apparatus according to this invention comprises:
a D/A converter;
control means for gradually varying a converted output voltage from the D/A converter from an initial voltage to a prescribed voltage when performing muting and gradually varying the converted output voltage from the prescribed voltage to the initial voltage when releasing the muting; and
muting means for passing an audio signal in such a manner that an impedance between an audio signal path and a ground level is increased when the converted output voltage becomes the initial voltage and interrupting passage of the audio signal in such a manner that the impedance between the audio signal path and the ground level is decreased when the converted output voltage becomes the prescribed value.
Specifically, the converted output voltage produced from the D/A converter varies gradually. Therefore, in the muting means, the impedance between the audio signal path and ground level varies gradually. Thus, occurrence of pop sound is prevented when the state of the muting means is changed from the state of interrupting the passage of the audio signal to the state of passing the audio signal.
In addition to the above construction, the D/A converter has an converting accuracy of at least 8 bits.
Specifically, although the converted output voltage from the D/A converter varies stepwise, the stepwise variation is minute so that it is substantially smooth.
In addition to the above construction, the initial voltage is set in the vicinity of 0 V,
the muting means includes a muting transistor with a collector connected to the audio signal path, an emitter connected to ground and a base supplied with the converted output voltage, and
the control means slows down the speed of reducing the converted output voltage when the converted output voltage is caused to fall to cross the vicinity of 0.6 V.
Specifically, when the converted output voltage falls to cross 0.6 V, the muting means is shifted from the state of interrupting the passage of the audio signal to the state of passing the audio signal. Therefore, the shifting from the state of interrupting the passage of the audio signal to the state of passing the audio signal becomes more gentle.
The audio muting apparatus comprises:
a D/A converter;
envelope detecting means for extracting an envelope signal from an output from the D/A converter;
control means for changing the state of the D/A converter between a first output state and a second output state at prescribed time intervals, the first output state being a state of producing a first voltage whereas the second output state being a state of producing a second voltage; and
muting means for varying the impedance between the audio signal path and ground level to interrupt the passage of the audio signal according to a level of the envelope signal.
The control means gradually varies a difference between the first voltage and the second voltage from the vicinity of 0 V to a prescribed voltage when performing muting and gradually varying the difference between the first voltage and the second voltage from the prescribed voltage to the vicinity of 0 V when releasing the muting.
Specifically, since the D/A converter is controlled at prescribed time intervals, the control is simplified. Where the muting is carried out and the muting is released, also when the D/A converter is changed between the first output state and the second output state at prescribed time intervals, the envelope detecting means produces an envelope signal whose level changes gently. Therefore, in the muting means, the impedance between the audio signal path and grounding level varies gently. As a result, occurrence of pop sound is prevented when the state of the muting means is changed from the state of interrupting the passage of the audio signal to the state of passing the audio signal.
In addition to the above construction, the period of changing the state of the D/A converter between the first output state and the second output is shortened.
Specifically, the signal component which must be removed by the envelope detecting means is a signal component at a high frequency. Therefore, the signal component which is not necessary can be removed using a capacitor having small capacitance.
In addition to the above construction, the period is set to shorter than 100 xcexcS.
Specifically, the signal component which must be removed in the envelope detecting means is a component at a frequency higher than 10 KHz. Therefore, the signal component which is not necessary can be removed using a capacitor having small capacitance.
In addition to the above construction, the audio signal path is a signal path through which a received audio signal of television broadcasting is passed, and
the control means performs muting when a received channel of television broadcasting is changed.
Specifically, when the channel received in the television broadcasting is changed, occurring noise can be muted.